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Data Link Control : HDLC & PPP in Data Communication and Networking

Data Link Control (DLC) ensures reliable data transfer over a communication link. HDLC (High-Level Data Link Control) is a bit-oriented protocol that provides error detection, flow control, and framing in synchronous networks. It is widely used in WANs due to its efficiency and reliability. PPP (Point-to-Point Protocol), on the other hand, is a byte-oriented protocol used for direct communication between two nodes. It supports authentication, compression, and error detection, making it ideal for dial-up and broadband connections. Both protocols play a crucial role in maintaining reliable data

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Data Link Control : HDLC & PPP in Data Communication and Networking

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  1. Data Link Control : HDLC & PPPinData Communication and NetworkingbyDr Anita ChoudharyAssistant Professor (NPIU)

  2. HDLC High-level Data Link Control (HDLC) is a bit-oriented protocol for communication over point-to-point and multipoint links. It implements the ARQ mechanisms we discussed in this chapter. Topics discussed in this section: Configurations and Transfer ModesFramesControl Field

  3. Figure 1 Normal response mode

  4. Figure 2 Asynchronous balanced mode

  5. Figure 3 HDLC frames

  6. Figure 4 Control field format for the different frame types

  7. Table 1 U-frame control command and response

  8. Example 1 Figure 5 shows how U-frames can be used for connection establishment and connection release. Node A asks for a connection with a set asynchronous balanced mode (SABM) frame; node B gives a positive response with an unnumbered acknowledgment (UA) frame. After these two exchanges, data can be transferred between the two nodes (not shown in the figure). After data transfer, node A sends a DISC (disconnect) frame to release the connection; it is confirmed by node B responding with a UA (unnumbered acknowledgment).

  9. Figure 5 Example of connection and disconnection

  10. Example 2 Figure 6 shows an exchange using piggybacking. Node A begins the exchange of information with an I-frame numbered 0 followed by another I-frame numbered 1. Node B piggybacks its acknowledgment of both frames onto an I-frame of its own. Node B’s first I-frame is also numbered 0 [N(S) field] and contains a 2 in its N(R) field, acknowledging the receipt of A’s frames 1 and 0 and indicating that it expects frame 2 to arrive next. Node B transmits its second and third I-frames (numbered 1 and 2) before accepting further frames from node A.

  11. Example 2 (continued) Its N(R) information, therefore, has not changed: B frames 1 and 2 indicate that node B is still expecting A’s frame 2 to arrive next. Node A has sent all its data. Therefore, it cannot piggyback an acknowledgment onto an I-frame and sends an S-frame instead. The RR code indicates that A is still ready to receive. The number 3 in the N(R) field tells B that frames 0, 1, and 2 have all been accepted and that A is now expecting frame number 3.

  12. Figure 6 Example of piggybacking without error

  13. POINT-TO-POINT PROTOCOL Although HDLC is a general protocol that can be used for both point-to-point and multipoint configurations, one of the most common protocols for point-to-point access is the Point-to-Point Protocol (PPP). PPP is a byte-oriented protocol. Topics discussed in this section: Framing Transition Phases Multiplexing Multilink PPP

  14. Figure 7 PPP frame format

  15. Note PPP is a byte-oriented protocol using byte stuffing with the escape byte 01111101.

  16. Figure 8 Transition phases

  17. Figure 9 Multiplexing in PPP

  18. Figure 10 LCP packet encapsulated in a frame

  19. Table 2 LCP packets

  20. Table 3 Common options

  21. Figure 11 PAP packets encapsulated in a PPP frame

  22. Figure 12 CHAP packets encapsulated in a PPP frame

  23. Figure 13 IPCP packet encapsulated in PPP frame

  24. Table 4 Code value for IPCP packets

  25. Figure 14 IP datagram encapsulated in a PPP frame

  26. Figure 15 Multilink PPP

  27. Example 3 Let us go through the phases followed by a network layer packet as it is transmitted through a PPP connection. Figure 17 shows the steps. For simplicity, we assume unidirectional movement of data from the user site to the system site (such as sending an e-mail through an ISP). The first two frames show link establishment. We have chosen two options (not shown in the figure): using PAP for authentication and suppressing the address control fields. Frames 3 and 4 are for authentication. Frames 5 and 6 establish the network layer connection using IPCP.

  28. Example 3 (continued) The next several frames show that some IP packets are encapsulated in the PPP frame. The system (receiver) may have been running several network layer protocols, but it knows that the incoming data must be delivered to the IP protocol because the NCP protocol used before the data transfer was IPCP. After data transfer, the user then terminates the data link connection, which is acknowledged by the system. Of course the user or the system could have chosen to terminate the network layer IPCP and keep the data link layer running if it wanted to run another NCP protocol.

  29. Figure 16 An example

  30. Figure 17 An example (continued)

  31. Thank You

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